:( sorry, I guess it's still quite demanding, too demanding, despite being a browser game

Are you using Chrome?

No, FF 52.9.0 x86_64 as shipped in debian 9.5.

That may well be why. There are tons of reports of FF being unusably slow even on the newest processors.

I'll see if I can try newer versions of FF when I get around to it.

Chrome, like all Google trash, is not an option [0].

[0] https://www.theregister.co.uk/2015/06/17/debian_chromium_hub...